This invention pertains to polybenzoxazole-based mixed matrix membranes and methods for making and using these membranes.
In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications have achieved commercial success, including carbon dioxide removal from natural gas and from biogas and enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex™ cellulose acetate polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.
The membranes most commonly used in commercial gas separation applications are polymeric and nonporous. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as PA) and the selectivity (αA/B). The PA is the product of the gas flux and the selective skin layer thickness of the membrane, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
Polymers provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for gas separation. A polymer material with a high glass-transition temperature (Tg), high melting point, and high crystallinity is preferred. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore let smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through more slowly as compared to polymers with less stiff backbones. However, polymers which are more permeable are generally less selective than are less permeable polymers. A general trade-off has always existed between permeability and selectivity (the so-called polymer upper bound limit). Over the past 30 years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success. In addition, traditional polymer membranes also have limitations in terms of thermal stability and contaminant resistance.
Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability. For example, UOP's early field practice found that polymer membrane performance can deteriorate quickly. The primary cause of loss of membrane performance is liquid condensation on the membrane surface. Condensation is prevented by providing a sufficient dew point margin for operation, based on the calculated dew point of the membrane product gas. UOP's MemGuard™ system, a regenerable adsorbent system that uses molecular sieves, was developed to remove water as well as heavy hydrocarbons from the natural gas stream, hence, to lower the dew point of the stream. The selective removal of heavy hydrocarbons by a pretreatment system can significantly improve the performance of the membranes. Although these pretreatment systems can effectively remove heavy hydrocarbons from natural gas streams to lower their dew point, the cost is quite significant. Some projects showed that the cost of the pretreatment system was as high as 10 to 40% of the total cost (pretreatment system and membrane system) depending on the feed composition. Reduction of the pretreatment system cost or total elimination of the pretreatment system would significantly reduce the membrane system cost for natural gas upgrading. On the other hand, in recent years, more and more membrane systems have been applied to large offshore natural gas upgrading projects. For offshore projects, the footprint is a big constraint. Hence, reduction of footprint is very important for offshore projects. The footprint of the pretreatment system is also very high at more than 10-50% of the footprint of the whole membrane system. Removal of the pretreatment system from the membrane system has great economical impact especially to offshore projects.
High-performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole have been developed recently to improve membrane selectivity, permeability, and thermal stability. These polymeric membrane materials have shown promising properties for separation of gas pairs such as CO2/CH4, O2/N2, H2/CH4, and propylene/propane (C3H6/C3H8). However, current polymeric membrane materials have reached a limit in their productivity-selectivity trade-off relationship. In addition, gas separation processes based on the use of glassy solution-diffusion membranes frequently suffer from plasticization of the stiff polymer matrix by the sorbed penetrant molecules such as CO2 or C3H6. Plasticization of the polymer as represented by the membrane structure swelling and significant increase in the permeabilities of all components in the feed occurs above the plasticization pressure when the feed gas mixture contains condensable gases.
On the other hand, some inorganic molecular sieve membranes such as SAPO-34 and carbon molecular sieve membranes offer much higher permeability and selectivity than polymeric membranes for gas separations, but are high cost, have poor mechanical stability, and are difficult for large-scale manufacture. Therefore, it is still highly desirable to provide an alternate cost-effective membrane with improved separation properties.
A recent publication in SCIENCE reported a new type of high permeability polybenzoxazole polymer membranes for gas separations (Ho Bum Park et al, SCIENCE 318, 254 (2007)). The polybenzoxazole membranes are prepared from high temperature heat treatment of hydroxy-containing polyimide polymer membranes containing pendent hydroxyl groups ortho to the heterocyclic imide nitrogen. These polybenzoxazole polymer membranes exhibited extremely high CO2 permeability (>1000 Barrer) which is about 100 times better than conventional polymer membranes and similar to that of some inorganic molecular sieve membranes but lower CO2/CH4 selectivity than that of some small pore inorganic molecular sieve membranes for CO2/CH4 separation.
Based on the need for a more efficient membrane than polymer and inorganic membranes, a new type of membrane, mixed matrix membranes (MMMs), has been recently developed. Mixed matrix membranes are hybrid membranes containing fillers, such as molecular sieves, dispersed in a polymer matrix.
Much of the research conducted to date on mixed matrix membranes has focused on the combination of a dispersed solid molecular sieving phase, such as molecular sieves or carbon molecular sieves, with an easily processed continuous polymer matrix. For example, see the following patents and published patent applications: U.S. Pat. No. 6,626,980; US 2005/0268782; US 2007/0022877; and U.S. Pat. No. 7,166,146. The sieving phase in a solid/polymer mixed matrix scenario can have a selectivity that is significantly larger than that of the pure polymer. Therefore, the addition of a small volume fraction of molecular sieves to the polymer matrix can increase the overall separation efficiency significantly. Typical inorganic sieving phases in MMMs include various molecular sieves, carbon molecular sieves, and traditional silica. Many organic polymers, including cellulose acetate, polyvinyl acetate, polyetherimide (commercially Ultem®), polysulfone (commercial Udel®), polydimethylsiloxane, polyethersulfone, and several polyimides (including commercial Matrimid®), have been used as the continuous phase in MMMs.
All the molecular sieve/polymer MMMs that have been reported in the literature, however, have either much lower permeability or much lower selectivity than the recently reported new type of high permeability polybenzoxazole polymer membranes for gas separations.
The present invention overcomes the problems of both the prior art mixed matrix membranes and inorganic molecular sieve membranes by providing a new type of high performance polybenzoxazole-based mixed matrix membrane and a route to make said membrane that has the following properties/advantages: high thermal stability, significantly higher selectivity than the neat polybenzoxazole polymer membrane, and much higher permeability than traditional mixed matrix membrane.